cmd: mtd: add 'mtd' command

There should not be a 'nand' command, a 'sf' command and certainly not
a new 'spi-nand' command. Write a 'mtd' command instead to manage all
MTD devices/partitions at once. This should be the preferred way to
access any MTD device.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Jagan Teki <jagan@openedev.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>